The present invention relates to a receiver amplifier and more particularly to an optoelectronic receiver circuit for accepting a low level, high impedance output signal from a photodiode and then amplifying this signal to a voltage and power level suitable for interfacing with standard circuitry while introducing a minimum of signal distortion.
One problem in transmitting information through standard electrical cables is the undesirable effect of radiated and conducted EMI electro-magnetic interference) normally associated with these cables. Such EMI may, for example, cause spurious or other erroneous readings from equipment attached to the cable. One solution to this problem has been to convert the information to be transmitted from electrical energy to light energy, transmit the light energy signal through a light conducting cable such as, for example, a fiber optic bundle, and reconvert the light energy into electrical energy at the receiving end. Since the light conducting cable is not responsive to EMI, interference on the line is thus eliminated.
Fiber optic technology is being considered for future applications in two distinct areas of the data communication field. The first of these application areas, which is presently under intensive development, is concerned with the transmission of data over medium to long distances. Long distance data transmission utilizes low loss optical fiber, avalanche photodiode detectors with special low noise preamplifiers and bias stabilization circuits, thermoelectrically cooled laser diodes, and periodic repeater stages. Because of the potentially large commercial application for long distance optical data transmission, much privately sponsored research is directed towards this area. Medium and long distance optical communication links are also of considerable interest to the military departments.
The second area of development is concerned with the optical transmission of data over short distances of a few hundred meters or less. The advantages to be gained over the use of conventional wire cables include: high per channel data rate capability, immunity to electro-magnetic interference, lower cable weight, elimination of fire hazard due to electrical shorting, and potentially lower cost. For short length data link applications, multi-fiber bundles of medium and high loss fiber are utilized. Light emitting diodes (LED's) are employed as optical sources, and photodiodes are used for optical detection. Short distance optical data transmission is of particular interest to the military departments since this technology has been proposed for the optical wiring of aircraft where line lengths of 150 feet or less are encountered.
The technology required for the application of fiber optic data transmission systems to military equipment is in the early feasibility stage of development. At present, considerable effort is being expended towards defining and developing the components and systems needed for the implementation of fiber optics data links. Much progress is still required in all aspects of fiber optics technology before reliable, large scale applications can be made to military systems. In view of the above, a very limited amount of prior art exists in the particular area of optoelectronic receiver design directly pertaining to the invention described in this paper. This receiver amplifier configuration is optimized for use in avionics applications. The use of short lengths of medium loss fiber, LED source diodes, and PIN photodiodes is assumed.
If there is a low to medium data rate requirement, then the optoelectronic receiver can generally be designed with dc coupling. Conversely, a high data rate system requires the use of wide-bandwidth, ac coupled amplifiers. The use of ac coupling introduces two circuit problems. First, the lack of low frequency response imposes a limit on the number of "ones" or "zeros" which can be sent in a continuous string when NRZ (non return to zero), binary coding is utilized. The second problem is caused by the variation in average dc output level with data word content. In order to avoid difficulties, careful design of the one-zero threshold detection circuitry is required, in conjunction with restrictions on the data word content. Both of these problems can be significantly reduced by the use of a Manchester type code which has a logic transition in each bit interval. Another advantage of this type of coding is that the clock signal can be recovered from the data word at the receiver. The primary disadvantage is that two bit times are required for each data bit transmitted. As a result, the usable upper data rate of the system is one-half of the rate at which simple binary coded information can be sent. In summary, with an ac coupled data link system there is an interdependence between the digital coding scheme employed and the hardware implementation. The use of a dc coupled system eliminates this interdependence.
In the design of a dc coupled receiver there is a tradeoff between the required dc gain sensitivity and bandwidth. For optimum dc sensitivity the sources of dc error must be reduced to a minimum. In general, a very high level of dc performance can be achieved only by use of components and circuit configurations which inherently have low bandwidth capabilities. Conversely, wide-bandwidth systems must utilize ac coupled amplifiers which have no dc or low frequency response.